I think I’m seeing variance handled incorrectly by...

# ksp

[elihart]

I think I’m seeing variance handled incorrectly by KSP, but could use a sanity check. For the built in List<out T> it should be covariant, and javac processors view it that way, but KSP reports the argument as invariant. I have a repro using xprocessing. I don’t think it is a problem within xProcessing because the debugger shows the originating KSTypeImpl to have an invariant argument

@Test
    fun testListVariance() {
        val libSource = Source.kotlin(
            "lib.kt",
            """
            class KotlinClass {
                fun foo(list: List<CharSequence>) {}
            }
            """.trimIndent()
        )
        runProcessorTest(listOf(libSource)) { invocation ->
            val clazz = invocation.processingEnv.requireTypeElement("KotlinClass")
            val param = clazz.getDeclaredMethods().single().parameters.single()

            if (invocation.isKsp) {
                expectThat(param.type.typeName.toString())
                    .isEqualTo("java.util.List<java.lang.CharSequence>")
            } else {
                expectThat(param.type.typeName.toString())
                    .isEqualTo("java.util.List<? extends java.lang.CharSequence>")
            }
        }
    }

[elihart]

The KSTypeImpl in KSP showing the argument as invariant

[Jiaxiang]

looks a bug to me

[Ting-Yuan Huang]

Could be related or not, but why it is java.util.List, instead of kotlin.collections.List?

[Ting-Yuan Huang]

On the other hand, could it be that param.type.typeName.toString() prints use-site variance instead of declaration-site variance?

[Jiaxiang]

in this case it should respect the declaration site variance.

[Ting-Yuan Huang]

The KSTypeImpl also looks good to me; The argument is invariant.

[Jiaxiang]

if you try to write List<in T>, compiler will complain about conflicting variance.

[elihart]

why it is java.util.List, instead of kotlin.collections.List

That’s the JavaPoet TypeName representation that xprocessing exposes. Since it is for java codegen it converts kotlin instrinsics to the java equivalent

writing List<out CharSequence> gives a warning because List already marks its type as out. so the type should always be covariant - it is inherited

[Ting-Yuan Huang]

Kotlin allows variance annotations to appear in both declaration and use sites. Type arguments (need to) hold use-site variances, instead of the combined (projected) information from type parameters. It is more like a difference in how Kotlin and Java models variance than a bug.

[elihart]

thanks for explaining. I see now that if I dig deeper into the type class I can access the declaration and the type arguments, which show the original type argument E as covariant.

It seems like a bug with XProcessing then, so I’ll file there

[elihart]

reported at https://issuetracker.google.com/issues/204415667 in case anyone else runs into this.

Thanks a lot for the help!

[Ting-Yuan Huang]

I think it should be feasible to write a small util to convert Kotlin variances into Java wildcards on top of KSP's API. Please let us know if you see any issues implementing it or feel it better to be implemented in KSP.